1. Field Of the Invention
The present invention is directed to a method for operating a fan beam computer tomography apparatus of the third generation, having a measurement unit, including an x-ray source and a detector array, which rotates around the system axis.
2. Description of the Prior Art
In fan beam computer tomography systems of the third generation, having continuous irradiation of the examination subject, the arrangement of the detector elements and the conduct of the scanning should be undertaken to optimize the factors of image quality, data acquisition and processing time, and radiation stress on the patient. These factors are, to a certain extent, competing in the sense that maximizing one factor may require adjustment of the other factors. In general, however, it is always desirable to prevent the generation of scan artifacts and one strives to balance the goals of obtaining an optimally high image quality (topical and contrast resolution) with a given total number of measured data, obtaining a prescribed image quality with an optimally low radiation stress on the patient, and conducting the tomographic scanning of a large volume with a given data acquisition and processing time.
A problem associated with computer tomography systems of the third generation is schematically illustrated in FIG. 4. FIG. 4 shows a schematic illustration of the arrangement of components in a known third generation computer tomography apparatus, with reference to an x-y coordinate system. The focus 2 of an x-ray source (not separately shown) rotates around a focus circle 1, centered at 8, and a fan beam 3 emanating from the focus 2 is incident on a detector array 4 after passing through a measurement field 5. The detector array 4 is composed of a series of detector elements 7, which rotates in combination with the focus 2. Each detector element 7 has a width b, and the detector elements 7 have a center-to-center spacing d, shown exaggerated in FIG. 4 for explanatory purposes. In computer tomography systems of this type, the highest spatial frequency B which arises in the network data is defined by the detector with b, i.e., B=(approximately) 1/b. According to the Nyquist criterion, however, a scanning rate of at least 1/2d is needed to avoid aliasing. This means that the spacing d between detector elements cannot be greater than b/2. This is not physically possible, and therefore aliasing is inherent in an apparatus of this type.
One solution which has been proposed to avoid or minimize aliasing in a third generation computer tomography apparatus is the so-called "beam addition 2 mode" as disclosed in European application 0 231 037. This approach is schematically illustrated in FIG. 5. In the "beam addition 2 mode" the respective measured values from two neighboring detector elements 7 are combined to form an "artificial" measured value. This is shown in FIG. 5, wherein an "artificial" detector element N' is shown as a combination of the neighboring detector elements n-1 and n, and an adjacent "artificial" detector element n'+1 is shown as a combination of neighboring detector elements n and n+1. This approach satisfied the Nyquist criterion, however, resolution is low because the artificial detectors have a larger width than the actual detector elements 7.
Another approach which avoids or minimizes the problem of aliasing in a third generation computer tomography apparatus is disclosed in the article "Sampling in Fan Beam Tomograph," Natterer, Westfaelishe Wilhelm--Universitaet Muenster, Institut fuer Numerische und Instrumentelle Mathematik, Dec. 19, 1991, appearing in SIAM J. Appl. Math. 1992. In contrast to the above techniques, which try to satisfy the Nyquist criterion within each fan projection, i.e., making d&lt;b as far as possible, Natterer discloses an approach wherein d within a projection is made much larger than b. This is schematically illustrated in FIG. 6. Natterer recognized that even though all detector elements 7 in the array 4 will have radiation incident thereon for each position of the fan beam, it is not necessary to use the data from every detector element per projection in order to construct the image. The Natterer approach is to use the data only from selected detector elements in the array within each projection, and these selected detector elements can be selected so as to be spaced from each other so that d within a projection is much larger than b. Of course, this means that far less data is obtained (or used) per projection. This means that many more projections must be undertaken in order to obtain (use) the measured values from each detector element. This is accomplished by employing an offset .gamma.=m.multidot..DELTA..beta., wherein m=a+i/M for i=0,1, . . . M-1, wherein A is an initial misalignment, and after M projections a is again reached.
Although the approach of Natterer is suitable for examining inanimate objects, it is not suitable for conducting examinations of human subjects, because of the high number of projections which are needed in order to obtain a complete data set from every detector. The necessity of conducting such a high number of projections would cause a human subject to be subjected to an unacceptably high radiation dose.
It is therefore a problem in the operation of third generation computer tomography systems to provide a method which avoids or minimizes aliasing, which is suitable for use in examining human subjects.